Field of the Invention
Method and apparatuses for automatic, real-time nondestructive testing (NDT) of composite, honeycomb, and gridlock structures using thermography to determine the presence, type, location, size, and strength of defects in the composite structure without having to visually interpret raw infrared (IR) images. The present invention is motivated by the types of composite structures found on aircraft, but the method and apparatuses apply to any structure comprised all or partially of composite materials such as wind blades, boats, cars, and building materials. The NDT method and apparatuses of the present invention are comprised of an IR camera, a heating or cooling method to change the temperature of the composite structure relative to the ambient temperature in such a way as to produce detectable IR intensity changes in the region of the defect, an electronic ruler to determine the location of the IR images on the composite structure, a computer, noise cancellation signal processing algorithms and software, and tabular and graphical output displays. The processing, location, and output displays can be applied to any NDT system that collects and processes a two dimensional amplitude images (e.g., x-ray systems).
Brief Description of Prior Art
Composite materials, such as boron, Kevlar, graphite, and carbon materials, are widely used in many advanced aircraft, automobile, ship, and construction structural systems. Reliable damage assessment for these structures, especially aircraft structures, which are subject to wear and tear, including extreme operational conditions, is one of the most serious and costly problems faced in the field of maintenance. This problem is compounded by the fact that damage can occur in many different forms. Reliable systems for detecting damage are particularly important for older and aging systems where structural failure may have significant losses of life or money.
Damage in composites can be detected in numerous ways, but the conventional detection methods are frequently limited to certain kinds of materials and structural geometries, and they are usually weak at quantifying the type of damage. In addition, these methods often require the structure to be at least partially disassembled so that a skilled technician can interpret the results of the inspection measurements. This increases the labor costs and adds to the time needed to complete the inspection. Improved inspection systems need to address defect signature analysis with a view to implementing an automated defect identification, extraction, and classification methodology (e.g., disbonds, delamination and water penetration into the honeycomb) that will ensure minimum operator intervention. The operator should only be needed to verify the presence of a defect by inspecting the IR video images manually after they are processed in real time using advanced signal processing methods.
The method and apparatuses of the present invention are motivated by the need for automatic, portable, easy-to-use, and low-cost methods of NDT inspection of the composite structures on aircraft, automobiles, ships, and construction that are fast, accurate, and reliable. Many of the NDT methods require highly skilled technicians to both perform the inspection and interpret the results of the inspection from visual analysis of raw sensor images. For these reasons, NDT inspection using thermography, which has been around for many years, has not gained the same acceptance and market share as some of the other methods. This is a problem, because thermography can detect and identify all of the defects of interest, which is not possible with many of the other methods.
The measurement benefits of thermography methods can be realized if the method can be more efficiently and accurately implemented. The spatial coverage and the ability to distinguish and identify between defects offers significant advantages over other NDT methods like x-ray, acoustic, and coin tap testing inspection methods. For example, thermography methods can detect disbonds and delamination in honeycomb composite aircraft structures, including beneath repaired and refurbished areas. Such measurements are not possible with x-ray methods. While x-ray methods can detect the presence of water or moisture in a honeycomb structure, they cannot differentiate the water or moisture from a buildup of epoxy that can be done with thermography methods. Acoustic methods, which require the application (and cleanup) of gel to the surface before making point measurements with a small probe are operationally messy and slow. Thermography methods cover larger spatial areas (vice point measurements) and do not require the application of gel or any preparation of the surface like paint removal. Thermography methods can also determine what the detected defect is (i.e., disbond, delamination, fluid ingress, epoxy), while other methods cannot (e.g., coin tap test).
FIG. 9 illustrates the output of a raw IR image obtained 30 s after heating the surface of an aircraft honeycomb boron composite used in the vertical stabilizer of the US Air Force (USAF) F-15 jet aircraft shown in FIG. 1. An illustration of a section of the honeycomb boron composite 20 is shown in FIG. 2. The boron composite 26 over the honeycomb 28 and the aluminum skin 24 over the honeycomb is shown. Distinct white spots in the IR image can indicate the presence of a disbond (separation of the boron composite from the aluminum honeycomb) or a delamination (separation between one or more layers of the boron composite). Visual inspection of the IR image indicates many such areas and significant ambiguity. FIG. 10 illustrates the results of the same IR image processed with a computer using a noise cancellation background removal method. One can easily see the disbond along the right edge of the aircraft structure and not be confused by the large area of ambiguous white spots along the left edge and center sections of the composite structure. This simple illustration shows the benefit of signal processing methods over visual interpretation in terms of accuracy and reliability. It minimizes the number of false detections (i.e., false alarms). It is also significant that the computer analysis is done automatically and can be done remotely without having to output the IR image.
The method and apparatuses of the present invention improve the performance, completely automate, and make it easy to use thermography for inspection of composite structures like those found on military and commercial aircraft. Skilled technicians are not needed for the operational implementation of the method in the field or for interpretation of the results. Such skilled interpretation can be reserved for validation and confirmation of the results. Because of the larger spatial coverage than other NDT techniques and the ability to determine the measurement coverage and locate defects on the composite structure itself automatically, thermography methods can be used to reduce the time and cost of an inspection.
The methods and apparatuses of the present invention also improve upon existing thermograph methods in terms of speed, accuracy, and cost because of the automation and signal processing. For example, automation is possible because of the noise cancellation, which effectively identifies and mitigates false alarms (false defects) and sources of interference from false targets like rivets and seams between materials.